Heads-up displays allow a user to view a scene while relevant information is overlaid on the scene, so that the user looking through the heads-up display simultaneously sees both the scene and the relevant information. For example, a pilot looking through a heads-up display while landing an airplane simultaneously sees the airport ahead (the scene) through the heads-up display while the heads-up display projects information such as speed, heading and altitude (the relevant information) that the pilot needs to land the plane.
In some uses of a heads-up display it can be useful to know what the user is viewing. One way to accomplish this is through ocular measurement technology, but existing technologies have some disadvantages. Among other things, existing ocular measurement technologies use an optical path separate from the optical path used for the display, making the heads-up display more bulky and complex and less streamlined.